The anticoagulant effects of warfarin, acenocoumarol, and phenprocoumon are increased by
co-trimoxazole (sulfamethoxazole with trimethoprim). Bleeding may occur if the anticoagulant dosage is not reduced appropriately.
The
incidence of the interaction with
co-trimoxazole appears to be high. If bleeding is to be avoided the INR should be well monitored and the warfarin, acenocoumarol, or phenprocoumon dosage should be reduced. Phenindione is said not to interact.